This invention relates to alarm indicators, and more particularly to an alarm signal generating circuit for providing an alarm output indicating a rapid decrease, or increase, in the rate of flow of a fluid.
In nuclear reactor installations, there is a need for rapidly and accurately detecting the onset of nuclear fuel motion, indicative of a breach in the reactor, so that the reactor may be shut down. Since nuclear fuel motion causes variation in the flow rate of the liquid coolant which is circulated through the reactor to cool and protect the reactor, when a liquid sodium coolant is used, the onset of fuel motion in the reactor may be detected by monitoring the flow rate of the coolant using a flowmeter. The amplitude and polarity of the flow signal generated by the flowmeter indicates the rate and direction of flow of the coolant. A rapid change in the flow rate is indicative of the onset of fuel motion.
However, the flow signal is a low level electronic signal, typically less than 10 millivolts, and is subject to long-term drift in real time. Also, the flow signal includes a 60 Hz component due to the large stray 60 Hz currents flowing in the annular linear induction pump which circulates the coolant. Known level alarm circuits are not capable of selecting rapid rates of decline, or rise, from a low-level electronic signal having appreciable noise, including 60 Hz pickup and long-term drift in real time, and providing an indication of the event in a relatively short time.